The present invention relates to a method of correcting information error in a device for recording and regenerating information to and from a disk-shaped recording medium, and more particularly relates to an error correction method suitable for a read only memory disk for recording data at the time of manufacturing the disk.
An optical disk has been used for a recording means for recording a large quantity of information. The Optical disk records and regenerates information by utilizing rotation of optical plane of polarization due to Kerr effect or Faraday effect or difference in refractivity due to difference in heating temperature. The optical disk records information with pits and projections at manufacturing time, and has a characteristic in that it can be easily manufactured as a non-rewritable read only memory disk and can regenerate information using a record regenerating device for rewritable disk. This characteristic makes it possible to distribute a large quantity of information with low price, and accordingly international standards for optical disk, for example, ISO, ECMA and so on, are established as an important technology in the multimedia era.
However, since the optical disk has a large error rate comparing to the conventional magnetic disk, use of error correcting code is a prerequisite. The types of error occurrence in optical disk can be classified into random error and burst error. The random error is caused by noise, and occurs by bit unit. The burst error is caused by faults, foreign objects or flaws in disk, and occurs by continuous bits. The sector in a 90 mm optical disk according to ECMA standard (ECMA-154) is composed of 512 bytes of user data, 4 bytes of vender unique data, 4 bytes of error detecting code by cyclic code (CRC; cyclic redundancy check character), 80 bytes of error correcting code (ECC) parity by Read-Solomon code. The error correcting code producing sequence is that the total of 520 bytes for the user data, vender unique data, CRC are divided 104 bytes each every 5 bytes, ECC parity of 16 bytes is produced and 5 sequences of error correcting sequences are formed in the whole sectors (5 interleave). The correcting capability of the error correcting code is up to 8 byte error in each correcting sequence for random error, and 40 byte error in the whole sectors. For burst error, the correcting capability is continuous 40 byte error per each sector. However, in a practical optical disk, a random error and a burst error are occurred at a time and the burst errors within correctable number steadily occur. If a random error occurs in such a state, the correction cannot be performed as the result even if the number of random errors is within correctable range. Therefore, the number of the steadily occurring burst errors must be decreased during data recording as few as possible. In this reason, in a rewritable disk device, when number of errors in the regenerating time of information just after recording exceeds a certain value even if the number is within the error-correctable number, the information is rewritten in another position. However, an ROM disk in which information is recorded by pre-bit at manufacturing time cannot be rewritten as described above. Therefore, one sector of parity sector is provided per 25 sectors according to ECMA-154 standard, and data placed in the same position in each sector is executed with exclusive-OR (EOR) every byte to record it to the parity sector. However, in this method, the recording efficiency is decreased since sectors for parity are separately required in addition to sectors for data. Further, correction cannot be often executed when the burst error and the random error occur at a time since one correcting sequence can correct only one byte. Therefore, a higher error correcting capability is required.